There has been used a technique for comparing (i) input image data obtained by reading a document image with a scanner and (ii) a pre-registered image, so as to determine similarity between the input image data and the pre-registered image, and controlling a process (such as copy, transmission, and edition) to the input image data in accordance with the result of the determination.
Examples of a method for determining similarity include: a method in which a keyword is extracted from an image with OCR (Optical Character Reader) so as to carry out matching with the keyword; a method in which a target image is limited to an image with ruled lines and matching is carried out based on features of the ruled lines (see Patent Document 1 (Japanese Unexamined Patent Publication: Tokukaihei 8-255236 (published on Oct. 1, 1996))); and a method in which similarity is determined based on distribution of color components of input image data and a reference image (see Patent Document 2 (Japanese Unexamined Patent Publication: Tokukaihei 5-110815 (published on Apr. 30, 1993))). Furthermore, Patent Document 3 (Japanese Unexamined Patent Publication: Tokukaihei 7-282088 (published on Oct. 27, 1995) discloses a technique in which a descriptor that is invariable to distortion caused by digitalization of an input document or to a difference between the input document and a document database is generated based on feature points of the input document, and matching between the input document and a document in the document database is performed using the descriptor and a descriptor database which stores descriptors and which indicates a list of documents including features from which descriptors are extracted. In the technique, when the descriptor database is scanned, votes for individual documents in the document database are accumulated, and a document with the largest number of votes or a document whose number of votes is over a certain threshold value is considered as a matching document.
Furthermore, Patent Document 4 (International Publication No. 2006/092957, pamphlet (published on Sep. 8, 2006) discloses a technique in which plural feature points are extracted from a digital image, a set of local feature points are determined out of the extracted feature points, a partial set of feature points is selected out of the determined set of local feature points, invariants relative to geometric transformation each as a quantity characterizing the selected partial set is calculated in accordance with plural combinations of feature points in the partial set, features are calculated by combining the calculated invariants, and a document or an image with the calculated features in a database is voted for, thereby searching a document or an image corresponding to the digital image.
Furthermore, an example of a technique for controlling a process for input image data in accordance with the result of similarity determination is as follows: in order to prevent forgery of a bill or securities by a color image forming apparatus, it is determined whether input image data is identical with image data such as a bill or securities (reference image) or not based on a pattern extracted from the input image data, and when the input image data is identical with the image data of the reference image, a specific pattern is given to an output image so as to specify the image forming apparatus that carries out copying, a copied image is daubed, or copying is prevented.
Incidentally, the image processing apparatus which determines whether the input image data is identical to image data of the reference image or not may give erroneous determination caused by conditions at the time of reading a document due to limitation such as document image reading accuracy and the like.
Thus, according to the aforementioned conventional techniques, in case where the input image data is determined as being identical to image data of the reference image, there are performed processes, such as addition of a specific pattern, restriction of processing by daubing a copy image, and forbidding of a copying operation. Thus, image processing desired by the user cannot be carried out with respect to input image data which is actually not identical to image data of the reference image but has been erroneously determined as being identical to the image data of the reference image.
Further, inversely with the aforementioned technique, it may be so arranged that image processing (e.g., copying, printing, electronic distribution, facsimile transmission, filing, image data correction/edition, and the like) is permitted in case where the input image data is determined as being identical to image data of the reference image and image processing is forbidden or restricted in case where the input image data is determined as not being identical to image data of the reference image. However, according to this arrangement, in case where the input image data is erroneously determined as being identical to image data of the reference image, image processing which actually should be forbidden or restricted is permitted. This may result in security problems such as leakage of secret information and a similar problem.